1. Field
The present invention relates to methods of membrane preparation to effect selective separation of solution components.
2. State of the Art:
In recent years, the technology surrounding membranes and their use, for example in the separation of solutes from solution, has grown from simple laboratory procedures to industrial processes having considerable technical and commercial impact, Lonsdale, H. K., J. Memb. Sci., 10 (1982), 81"181. Membranes are used on a large scale for many applications, among which are to produce potable water from sea water by reverse osmosis, to clean industrial effluents, to recover valuable constituents of solutions by electrolysis, and to effect various medical purposes.
Membrane chemistry and technology is interdisciplinary. On-going research strives to improve past processes which concern the selectivity and functionality of membranes. For example, Nakao et al. U.S. Pat. No. 4,909,810 discloses a vapor permselective membrane in the form of an ion exchange film made of a fluorine-containing polymer. Linder et al. U.S. Pat. No. 5,039,421 discloses a composite membrane for separating at least one dissolved component from another.
Transport of a solution through a membrane, normally for the purpose of separating at least one solute from a carrier liquid, is often accomplished by establishing a pressure differential at opposite faces of the membrane, whereby hydraulically driven permeation of the liquid solution is effected.
It can be shown thermodynamically that pressure differentials between the feed side or upstream face and the permeate side or downstream face of the membrane cause flux of the liquid components through the membrane. Such pressure differentials are related to membrane permeability by a well-known mathematical formula defining hydrostatic permeability (P): namely, P=Jl.DELTA..PHI., where J=permeant flux in appropriate units, l=membrane thickness, and .DELTA..PHI.is the difference in hydrostatic pressure between the feed side and permeate side of a membrane. Pressure is the most common driving force and accounts for several types of separation among which are pervaporation (PV), ultrafiltration (UF), reverse osmosis (RO) and microfiltration (MF). Although RO and PV are partially pressure and partially concentration driven, UF and MF are totally pressure driven.
Pressure differentials have frequently been produced by flowing a pressurized stream of a solution to be processed into the feed side of the membrane and less often by the additional use of a vacuum on the permeate side of the membrane. Hassett U.S. Pat. No. 5,131,266 suggests that a means for pervaporating an organic solvent may comprise a vacuum applied to the permeate side of a hollow tube membrane.
To date, flux rates have remained of moderate rate and selectivity of membrane permeability still remains problematic. The present invention provides improvement in the control of permeate rate of the flux, and in the selectivity of membrane permeability of solvents and solutes.